Model for activity coefficients and solubilities of Na2CO3 and Na2SO4 in high-temperature solutions

Abstract: Scaling of sodium sulfate and sodium carbonate double salts has long been recognized as a problem in the black liquor evaporation process. These salts are abundant in black liquor and often tend to precipitate on heat transfer areas, thereby significantly decreasing the heat transfer efficiency. It is therefore somewhat surprising that the detailed solubility and precipitation behavior of these salts have been studied by relatively few researchers. Several papers with experimental solubility data for the Na2CO3-Na2SO4 system can be found in the literature; there are, however, no studies presenting a consistent, thermodynamically-sound model for solubility and solid phase composition. This work uses published solubility and solid phase composition data from high-temperature measurements of the system Na-CO3-SO4-H2O in order to develop a model for predicting the solubility and solid composition within the double salt precipitation region. The model, based on a thermodynamic definition of the solubility constant with Pitzer ionic activities in the solution, has a form of a two-parameter empirical parabola. It is capable of calculating the solubility and the composition of the precipitate within the region of burkeite-sodium carbonate solid solution. Its application is limited to temperature regimes from 100-115°C and 150°C. Pitzer activity coefficient calculation has also been successfully applied to develop a method for predicting boiling point elevation of solutions containing Na2CO3, Na2SO4, NaOH, and Na2S.